HIV infection is associated with severe perturbations in the T cell receptor repertoire. In preliminary studies of CDR3 lengths of the CD8 T cell receptor beta chains (TCR BV) in 22 HIV infected children we have identified distinct patterns of clonal dominance. Interestingly, a pattern of major clonal dominance involving greater than or equal to 3 BV families was associated with preservation of the immune system (CDC Immune category 1 in 8 of 10 children) whereas 10 of 10 children with minor/none clonal expansions were in immune category 3. This grouping held true regardless of age at study and virus load. The majority of these BV clones expressed CD28, a molecule that protects cells from apoptosis. These findings, together with the demonstration in adults that clonal dominance in primary infection prevents CD4 cell decline have led us to hypothesize that a pattern of major CD8 clonal dominance in CDR3 lengths of T cell receptor repertoire is indicative of a robust immune response, that the HIV-specific cells in these dominant clones can be efficiently tracked by their ability to bind antigens and that resistance of these clones to activation induced apoptosis slows disease progression. In this proposal we will study the TCR BV repertoire by multiplex PCR assay for CDR3 lengths in a cohort of well characterized HIV infected children. We will use multiparameter flow cytometry to characterize the dominant BV clones in these children. Assays will include the powerful new technology for identifying antigen specific T cells with HIV gag HLA-2/Ig molecules in the dominant clones, investigation of the nature of the dominant clones with respect to their detailed phenotype and functional properties as determined by spontaneous and antigen-induced activation, cytokine secretion, chemokine secretion and cytolytic and noncytolytic anti viral properties. Lastly we will determine the replicative history of the cells by a flow based FISH assay and determine proliferation in vitro in conjunction with susceptibility for apoptosis using a nuclear dye DAPI together with surface labels including Annexin for apoptosis. We believe that these studies conducted longitudinally in a well characterized cohort of children will provide insights into the nature of the dominant CD8 clones in relation to disease progression and response to therapy.